@Debra300
I believe in London it is the remit of the local councils but, when I couldn't find any information on my council's website. I did look quite a while ago, so maybe there is something there now. How they enforce it, I have no idea, but I believe this rule is designed more to stop large-scale property developers with many listings etc. from taking over the rental market. We do have a housing crisis in London. So, I don't think the councils would be particularly interested in small-scale hosts renting out one property, but I could be wrong.
However, Airbnb DO enforce this. If you have an entire unit listing in London, as soon as you reach 90 nights of bookings, the rest of the year's calendar will be blocked. I had a friend who stopped renting his house on Airbnb because of this.
Of course, that's not to say you couldn't take 90 days of bookings on Airbnb and then try to fill the rest of the calendar using other listing sites. I doubt Airbnb would be double checking that. They have implemented the 90 day block on their system and therefore can say they have complied with the city's regulations so that box is ticked.